It is known to those skilled in the art that ethers, including unsymmetrical ethers, may be prepared by reacting an alcohol with another alcohol to form the desired product. The reaction mixture, containing catalyst and/or condensing agent may be separated and further treated to permit attainment of the desired product. Such further treatment commonly includes one or more distillation operations.
Methyl tert-butyl ether is finding increasing use as a blending component in high octane gasoline as the current gasoline additives based on lead and manganese are phased out. Currently most commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol (Eq. 1), catalyzed by a cationic ion-exchange resin (see, for example: Hydrocarbon Processing, October 1984, p. 63; Oil and Gas J., Jan. 1, 1979, p. 76; Chem. Economics Handbook-SRI, September 1986, p. 543-7051P). The cationic ion-exchange resins used in MTBE synthesis normally have the sulphonic acid functionality (see: J. Tejero, J. Mol. Catal., 42 (1987) 257; C. Subramamam et al., Can. J. Chem. Eng., 65 (1987) 613). ##STR1##
With the expanding use of MTBE as an acceptable gasoline additive, a growing problem is the availability of raw materials. Historically, the critical raw material has been isobutylene (Oil and Gas J., Jun. 8, 1987, p. 55), however, recently, U.S. Pat. Nos. 5,099,072, 5,169,592 and 5,081,318, inter alia, assigned to Texaco Chemical, disclose a one-step method for producing methyl tert-butyl ether (MTBE) from t-butanol (tBA) over various catalysts. It would be advantageous to obtain additional conversion of the t-butanol in the crude feedstock without having to recycle unconverted tertiary butanol.
The preparation of methyl tert-butyl ether from methyl and tert-butyl alcohols is discussed in S. V. Rozhkov et al., Prevrashch Uglevodorodov, Kislotno-Osnovn. Geterogennykh Katal. Tezisy Dokl. Vses Konf., 1977, 150 (C. A. 92:58165y). Here the tBA and methanol undergo etherification over KU-2 strongly acidic sulfopolystyrene cation-exchangers under mild conditions. This reference contains data on basic parameters of such a process. It is also noted that, although a plant for etherification over cation exchangers does not present any major problems, considerations include the fact that recycling large amounts of tert-butyl alcohol and methanol, as well as isobutylene, causes the scheme to be somewhat more expensive. Also, the progress of the reaction over cation exchangers is usually complicated by various adsorption and diffusion factors, by swelling phenomena, and by the variable distribution of the components between the solution and ion-exchanger phase. Furthermore, said acidic cation-exchangers with an organic (polystyrene or polymethacrylate) backbone generally have a very limited stability range with regard to operating temperatures, with temperatures above 120.degree. C. normally leading to irreversible destruction of the resin and loss of catalytic activity.
"Preparation of Methyl Tert-Butyl Ether (MTBE) Over Zeolite Catalysts" is an article by Pochen Chu and Gunther H. Kuhl, Ind. Eng. Chem. Res., 26, 365, 1987. Chu et al. disclose work which identifies ZSM-5 and ZSM-11 to be selective for the preparation of MTBE. Compared to the conventional Amberlyst 15 resin, the zeolites are thermally stable, and give no acid effluent; they provide high selectivity to MTBE with little or no diisobutene yield, are less sensitive to the CH.sub.3 OH/i-C.sub.4 H.sub.8 ratio and exhibit good selectivity even at ratios approaching unity. They provide high MTBE output, despite the unfavorable thermodynamic equilibrium, since the process utilizing these zeolites can be operated at high temperature and high space velocity. In addition, deactivation is not observed in the present short catalytic tests and reactivation is not required. The excellent selectivity of these two zeolites is believed to be effected by the size of their pore structure, which provides easy access to methanol and restricted access to isobutene. Zeolite beta was also tested, giving the poorest results and small pore zeolites were inactive. As expected, large pore zeolites do not exhibit shape selectivity.
An extensive body of knowledge of zeolite properties and catalytic potential has developed in recent years. A variety of different types of zeolites is known in the art, including natural and synthetic zeolites. Research has opened up a spectrum of new opportunities in the field of molecular shape selective catalysis, where the intracrystalline space accessible to molecules has dimensions near those of the molecules themselves. This field is discussed in an article titled "Molecular Shape Selective Catalysis", P. B. Weisz, New Horizons in Catalysis, Part A, 1980. For example, it is possible to catalyze the dehydration of n-butanol over a Linde 5.ANG. zeolite without reacting isobutanol which may be present. Such research has led to the concept of molecular engineering.
Early work in molecular engineering was very limited by the choice of zeolites. This limitation lead to the discovery of methods for zeolite synthesis, using large organic cations as templates in place of the traditional all-inorganic ionic species. This research opened the way to the synthesis of many new zeolites. Now a number of industrial processes, including selectoforming, M-forming, dewaxing, xylene isomerization, ethyl benzene production, toluene disproportionation and methanol-to-gasoline are based on shape selective zeolites. Since the early demonstrations of product selectivity, many more cases have been observed and many reviewed and reported by Csicsery and Derouane. See S. M. Csicsery,"Zeolite Chemistry and Catalysis", ACS Monograph 171, J. A. Rabo, Ed., American Chemical Society, Washington, D.C. (1976), and E. G. Derouane, "Diffusional Limitations and Shape Selective Catalysis in Zeolites", from Intercalation Chemistry, M. S. Whittinham, A. J. Jacobson, Eds., Academic Press, New York.
The natural crystalline aluminosilicate zeolites can be represented by the empirical formula: EQU M.sub.2 /.sub.n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
The synthetic X and Y type zeolites have framework structures similar to that of the natural mineral faujasite although they are distinct species. The unit cells are cubic with a cell dimension of nearly 25.ANG.. Each unit cell contains 192 SiO.sub.4 and AlO.sub.4 tetrahedra that are linked through shared oxygen atoms. See "Molecular Sieve Catalysis", J. W. Ward, Applied Ind. Catal., Vol. 3, 1984.
In the Y zeolites the three-dimensional framework comprising a tetrahedral arrangement of connected truncated octahedral provides giant supercages approximately 13.ANG. in diameter with eight supercages per unit cell. The supercages are interconnected by twelve-membered rings of about 8.ANG. in diameter. Many different chemical species can be absorbed by this large-pore system. Ibid., p. 275.
Various zeolites have characteristic structures which favor certain types of reactions. For instance, mordenite is one of the most silica-rich zeolite minerals, having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 10. The structure consists of chains of tetrahedra cross-linked by the sharing of oxygen atoms. Mordenite has high thermal stability, probably due to the presence of the large number of five-membered rings that are energetically favored. The dehydrated structure has a two-dimensional channel system accessible to small molecules, but not to typical hydrocarbon molecules. Ibid., pp. 275-6.
Erionite is probably the smallest pore zeolite used commercially.
A number of zeolites have been synthesized that have SiO.sub.2 /Al.sub.2 O.sub.3 ratios greater than 10 or are essentially pure silicas. Examples of those which have found commercial utility because of their shape selective properties are ZSM-5 and ZSM-11. Some of these zeolites are aluminum-free silicalites which have no ion-exchange properties and should properly be regarded as molecular sieves.
The Ward reference, ibid, offers a review of molecular sieve zeolites used in catalysis. Though zeolites have been known for a long time, the major stimulus in molecular sieve science came with the first synthesis of A zeolite by Milton, described in U.S. Pat. No. 2,882,243 (1959).
Zeolite molecular sieves can be modified by treatment by cation exchange, thermal or hydrothermal treatment and chemical modification. Most catalytic preparations involve an ammonium ion exchange, typically by refluxing the zeolite with at least a five-fold excess of aqueous ammonium salt.
Divalent cation exchange with elements such as calcium and magnesium is considered rather difficult according to Ward.
Rare earth ion exchange zeolites have played an important role in zeolite catalysis, particularly in fluid cracking catalysts and require multiple batch exchanges at elevated temperatures with excess solutions. Ibid., pp. 288-289.
Zeolites having higher silica/alumina ratios are more stable and, therefore, more suitable for treatment. Careful acid treatment can result in up to 75% of the alkali metal ions being replaced before structural collapse occurs. Ibid., p. 290.
The thermal or hydrothermal treatment of zeolites is also known. Thermal treatment of synthesized X and Y zeolites has no structural effects on the zeolite until the decomposition temperature of about 800.degree. C. is reached. It is possible to exchange and reexchange ions. For instance, it is possible to exchange with ammonium ions, calcine and exchange with rare earth. Ibid., p. 292.
Zeolites lose physically bound water on heating to about 150.degree. C. and exotherms around 800.degree. C. represent structural collapse of the zeolite. The hydroxyl groups are believed to be in different parts of the structure, some in supercages and some inaccessible to most absorbing molecules.
Zeolites can be modified to remove alumina by treatment with chelates such as acetylacetone and ethylenediamine tetraacetic acid. Aluminum atoms can be replaced with silicon tetrachloride or treatment with ammonium fluorosilicate. Ibid., p. 298.
A number of commercial applications of these synthetic zeolites are discussed in "Synthetic Zeolites in Commercial Applications", R. G. Muller, et al., SRI PEP Review v. 81-3-3 (1982). Due to the unique structure of zeolites and to the knowledge available today regarding properties and manufacturing processes, many uses have been discovered for zeolites in adsorbent and catalytic applications. Some of the reactions for which synthetic zeolites have been shown to be active catalysts include xylene isomerization, naphtha isomerization, light olefin oligomerization, toluene dealkylation, benzene hydrogenation, olefin and fat hydrogenation, methanation, dehydrogenation of ethylbenzene, dehydrohalogenation, desulfurization and propylene carbonylation.
Another good reference for familiarization with the relationship between molecular shapes, structures of zeolites and selectivity for certain catalysis is an article titled "Industrial Application of Shape Selective Catalysis", N.Y. Chen et al., Catal. Rev.-Sci. Eng., 28 185 (1986).
The zeolites of interest to shape-selective catalysis may be divided into three major groups according to their pore/channel systems. The first group includes 8-membered oxygen ring systems such as, for example, Linde A, erionite, chabazite, zeolite alpha, ZK-4, ZK-21, ZK-22 and several other less common natural zeolites.
The second group includes 10-membered oxygen ring systems such as, for example, ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-48 and laumontite, which has a puckered 10-membered oxygen ring. The rest of the medium pore zeolites are usually synthetic in origin; they are sometimes known as pentasils. They have a predominance of silicon.
The third group of zeolites is those having dual pore systems which have interconnecting channels of 12- and 8-membered oxygen ring openings. Examples include mordenite, offretite, clinoptilolite, ferrierite, etc.
There is a survey of aluminosilicate catalyst reactivity and of reactions catalyzed by aluminosilicates in an article titled "Clays, Zeolites and other Microporous Solids for Organic Synthesis," by John M. Thomas et al., in Modern Synthetic Methods, 1989, Vol. 5, p. 249.
It is stated at page 263 that the valency or the size of the exchangeable cation can be adjusted, thus fine-tuning the molecular sieving and shape-selective properties. For example, in the Na.sup.+ form of zeolite-alpha, the effective void space within the zeolite can be enlarged by replacement of Na.sup.+ by Ca.sup.2+ ions.
Dealumination of a zeolite can enhance the microporosity of a zeolite by increasing the Si/Al ratio of the anionic framework and can be represented by the following: ##STR2##
Hydrothermal treatment Will result in ultra-stabilization of the zeolite and recent work indicates Si:Al ratios can be increased to greater than 1000.
More than 40 distinct species of zeolite materials have been identified and there are at least 130 synthetic species. The pore sizes and compositions of typical commercially available zeolites are shown in Table A.
TABLE A ______________________________________ COMMERCIALLY AVAILABLE ZEOLITES Sorportion Pore Composition Capacity (wt %) Zeolite Size/nm Si/Al Cation H.sub.2 O nC.sub.6 H.sub.14 C.sub.6 H.sub.12 ______________________________________ Faujasite X 0.74 1-1.5 Na 28 14.5 16.6 Y 0.74 1.5-3 Na 26 18.1 19.5 US-Y 0.74 &gt;3 H 11 15.8 18.3 A 0.3 1.0 K, Na 22 0 0 A 0.4 1.0 Na 23 0 0 A 0.45 1.0 Ca, Na 23 12.5 0 Chabazite 0.4 4 * 15 6.7 1 Clinoptilite 0.4 .times. 0.5 5.5 * 10 1.8 0 Erionite 0.38 4 * 9 2.4 0 Ferrierite 0.55 .times. 0.48 5-10 H 10 2.1 1.3 L-type 0.6 3-3.5 K 12 8 7.4 Mazzite 0.58 3.4 Na, H 11 4.3 4.1 Mordenite 0.6 .times. 0.7 5.5 * 6 2.1 2.1 Mordenite 0.6 .times. 0.7 5-6 Na 14 4.0 4.5 Mordenite 0.6 .times. 0.7 5-10 H 12 4.2 7.5 Offretite 0.58 4 K, H 13 5.7 2.0 Phillipsite 0.3 2 * 15 1.3 0 Silicalite 0.55 ** H 1 10.1 0 ZSM-5 0.55 10-500 H 4 12.4 5.9 ______________________________________ *Denotes a mineral zeolite: cations variable, but usually Na, K, Ca, Mg **Very large Si:Al
Another important aspect regarding zeolites involves methods of generating acidity. Several ways of introducing acidity into a zeolite are known in the art and they result in the formation of Bronsted acid sites. The total acidity of a zeolite catalyst depends on both the concentration of acidic sites and the strength of the individual sites. The number and nature of active sites in a zeolite catalyst can be determined in several ways, including .sup.27 Al solid state NMR, uptake of base and poisoning experiments. Maximum overall acidity is often found for Si/Al ratios in the range of 5 to 20. Bronsted acid sites formed by various methods can form Lewis acid sites by dehydroxylation: ##STR3##
Where shape selectivity by size exclusion is the key to zeolite function, it can be accomplished either through reactant selectivity or product selectivity. It is believed Columbic field effects may also play a part. Another phenomenon which has been observed to contribute is configurational diffusion which occurs in situations where structural dimensions of the catalyst approach those of molecules; even subtle changes in dimensions of molecules can result in large changes in diffusivity, see Chen et al., Catal. Rev.-Sci. Eng., supra, p. 198.
Another type of selectivity which has been observed is spatiospecificity or restricted transition state, where both the reactant molecule and the product molecule are small enough to diffuse through channels, but the reaction intermediates are larger than either the reactants or the products and are spatially constrained. This is one of the most important properties of ZSM-5. Some zeolites, such as ZSM-5, ferrierite, cliniptilolite, offretite and mordenite have intersecting channels of differing channel size and may exhibit the phenomena of traffic control, see Ibid., p. 198.
Reactants which are of particular interest in shape selective catalysis include straight-chain and slightly branched paraffins and olefins, naphthenes and aromatics.
A concise and informative review of zeolite applications in organic synthesis is titled "Zeolite Catalysts Face Strong Industrial Future" in European Chemical News, Jul. 10, 1989, p. 23. Points worth noting are that the major success of zeolite catalysis is in catalytic cracking with about 300,000 tons per year being used worldwide. Typically these are zeolite-Y, (12-ring window) but oftentimes a medium-pore, such as H-ZSM-5, is added to increase aromatic content. The ZSM-5 consists of two pore systems (5-6.ANG. in diameter) which intersect to give spatial regions of around 9.ANG. diameter at the intersections.
Early work emphasized the role of medium-pore zeolites for aromatics substitution, but more recently, there has been extensive work describing the role of zeolites in the synthesis of organic intermediates containing oxygen and nitrogen, Ibid, p. 24.
In U.S. Pat. No. 4,214,307 to C. D. Chang et al. (Jul. 22, 1980), it is shown that hydration of C.sub.2 to C.sub.4 olefins to alcohols can be carried out over ZSM-5 below about 240.degree. C. and 10 to 20 atmospheres of pressure without forming ethers or other hydrocarbons, however, above 240.degree. C. propene and butenes undergo other olefinic reactions, forming higher molecular weight hydrocarbon products.
U.S. Pat. No. 4,058,576 to Chang et al. teaches the use of (pentasil-type) aluminosilicate zeolites, such as ZSM-5, having a pore size greater than 5 angstrom units and a silica-to-alumina ratio of at least 12, to convert lower alcohols to a mixture of ethers and olefins.
In U.S. Pat. No. 4,943,545, to Chang et al., there is suggested modification of Y-zeolites having Si:Al ratio of at least 4 with a very dilute (0.001.fwdarw.0.1N) solution of HF in a cracking catalyst as a means of reactivation.
U.S. Pat. No. 4,605,787 discloses a process for the preparation of methyl tert-butyl ether which comprises reacting methanol and isobutylene in vapor phase in the presence of ZSM-5 or ZSM-11 acidic zeolite catalyst.
U.S. Pat. Nos. 5,099,072; 5,169,592 and 5,081,318, assigned to Texaco Chemical Co., relate to various zeolite catalysts used in the one-step synthesis of methyl t-butyl ether from t-butanol.
In U.S. patent application Ser. No. 07/745,777 there is disclosed a catalyst for synthesis of MTBE from t-BuOH comprising hydrogen fluoride modified zeolites.
In U.S. patent application No. 07/917,885, there is disclosed a catalyst for MTBE synthesis comprising fluorophosphoric acid-modified zeolites.
In the art, where the feedstock for producing MTBE is t-butanol, the conversions are not as high as would be desirable. There does not seem to be any disclosure of a second-stage etherification, wherein bottoms from the primary reactor are reacted in a secondary etherification unit to obtain higher overall conversions. This would allow more complete conversion of t-butanol without the necessity of recycling. The second stage of such a two-step process would require a catalyst which can withstand very high temperatures.
It would be a substantial advance in the art if crude t-butanol/methanol feedstocks could be used and still obtain higher tBA conversions (&gt;90%) and total MTBE+isobutylene selectivity as high as 99%.